Spectroscopic Evidence for Intermolecular M−H···H−OR Hydrogen Bonding:  Interaction of WH(CO)₂(NO)L₂ Hydrides with Acidic Alcohols

Intermolecular hydrogen bonding of acidic alcohols (PhOH, (CF 3 ) 2 CHOH (HFIP), (CF 3 ) 3 CHOH (PFTB)) to the hydride ligand of WH(CO) 2 (NO)L 2 (L = PMe 3 ( 1 ), PEt 3 ( 2 ), P(O i Pr) 3 ( 3 ), PPh 3 ( 4 )) has been observed and characterized by IR and NMR spectroscopy in hexane, toluene- d 8, and CD 2 Cl 2 solutions. The H-bonding is an equilibrium process with medium −Δ H ° of 4.1−6.9 kcal/mol; the enthalpy increases on going from 4 to 1, i.e., the strongest bonding is found for the smallest and the most basic L = PMe 3 . The value of −Δ H ° depends on the p K a of the proton donors, increasing as the acidity does (PhOH < HFIP < PFTB). The IR and NMR data suggest C 2 v symmetry around tungsten in the ROH···HW(CO) 2 (NO)L 2 adduct, with the H···H distance of 1.77 Å (L = PMe 3 ) estimated from the hydride T 1min relaxation time. The relevance of the hydrogen bonding to the mechanism of protonation of metal hydrides is suggested.